1. Field of the Invention
This invention relates to frequency control circuits for a video signal and more particularly to an apparatus for controlling the frequency of a voltage controlled oscillator for use in, for example, a video tape recorder (VTR).
2. Description of the Prior Art
In the color signal processing circuit of a VTR, a carrier chrominance signal (with frequency of 3.58 MHz) is frequency-converted to a low frequency converted chrominance signal (with frequency fs=743 KHz) and then recorded on a magnetic tape, while upon playback, the reproduced low frequency converted chrominance signal is frequency-converted to a reproduced carrier chrominance signal (with frequency fc=3.58 MHz), thus making the usable frequency band narrow. In the prior art of the frequency conversion like the above, a VCO (voltage controlled oscillator) is frequency-controlled on the basis of a horizontal sync (synchronizing) signal separated from an input video signal and the frequency conversion is carried out by the frequency output signal derived from the VCO which is controlled as above; thus, the frequency converted output can be produced on the basis of the frequency signal which is locked in the frequency of the horizontal synchronizing signal. This frequency control circuit is a so-called AFC (automatic frequency control) circuit.
When the VCO is used to carry out the frequency conversion by the use of the frequency signal locked in the frequency of the horizontal synchronizing signal, if the frequency variable range of the VCO is wide, for example, the oscillation frequency of the VCO is largely different from the frequency of the horizontal synchronizing signal, upon starting, the oscillation frequency of the VCO may fail to lock onto the frequency of the horizontal synchronizing signal. To remove this shortcoming, it is proposed in the prior art to use a VCO which is adjusted so that its frequency variable range is set within a predetermined narrow range.
However, with such previously proposed VCO, when it is formed on an IC (integrated circuit), if the paramenter thereof is scattered, the change of the oscillation frequency of the VCO can not be restricted within a predetermined range so that the manufacturing yield thereof inevitably grows worse.